An occupant detection system including a capacitance type sensor and an occupant detection electronic control unit (ECU) is proposed in JP-A-11-271463. The capacitance type sensor outputs current or voltage signals indicating distortions of a weak electric field occurs around electrodes. In this system, a pair of electrodes are arranged in a seat. Only air is present between the electrodes when the seat is vacant. A child seat is present between the electrodes when it is mounted to the seat. A human body is between a person sits in the seat.
A specific inductive capacities of air, the child seat, and the human body are about 1, 2 to 5, and 50, respectively. The specific inductive capacity of the child seat depends on its material. The specific inductive capacities differ from each other, namely, capacitances between the electrodes differ from each other according to kinds of objects present between them.
The occupant detection ECU determines conditions of the seat based on the capacitance differences. Namely, it determines that the seat is vacant, equipped with a child seat, or occupied by an adult based on the capacitance differences. An air bag ECU determines permission or prohibition of air bag deployment based on the seat condition determined by the occupant detection ECU. It prohibits the air bag deployment when the seat is vacant or equipped with a child seat, and permits it when the seat is occupied by an adult.
A specific inductive capacity of water is about 80 and larger than that of the human body. Thus, it is difficult to distinguish between a condition in which the seat is occupied by an adult and a condition in which the seat is vacant when the seat is wet. An occupant detection system using a capacitance type sensor in combination with a weight sensor is proposed in JP-A-2003-520344 to improve the occupant detection accuracy in a condition that the seat is wet. For the same purpose, an occupant detection system using a capacitance sensor in combination with a weight sensor or a thickness sensor is proposed in JP-A-2002-36929. The weight sensors and the thickness sensor are less likely to be influenced by wetness in the seat. Therefore, the systems can determine whether the seat is wet. However, the system require additional sensors, that is, the weight sensors or the thickness sensor, and complexity and manufacturing cost of the systems increase.
An occupant detection system in which outputs of a capacitance type sensor are detected in chronological order is proposed in JP-A-2002-221579. The outputs vary differently in chronological order when a seat is occupied by an adult and when the seat is wet. This system determines whether the seat is wet based on the different chronological variation patterns. However, circuit configurations of an occupant detection ECU become complex.